Degradation of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 53

S

O

S

O

H 3 CO

H 3 CO

SH CH 2

P

CH 2

P

S N S

H 3 CO

OH

H 3 CO

N S

Hydrolysis

N

Methiodothion N

OCH 3

OCH 3

Chlorpyrifos undergoes hydrolysis easily in alkaline medium (Gilani et al. 2010).

Cl

Cl

S

S

Cl

Cl

SC 2 H 5

SC 2 H 5

Alkaline
Hydrolysis

O P

P

Cl + HO

N

SC 2 H 5

SC 2 H 5

OH

Cl

N

3.2.4 Carbamate Pesticides

Oxamyl and methomyl, widely used oxime pesticides, undergo rapid degradation in
anoxic and suboxic soil suspensions by a redox pathway involving Fe(II). During the
mechanism, Fe(II) forms a precursor complex that forms nitrile and thiol (Strathman and
Stone 2000).

R

O O

Fe (II)

R

Fe (II) + S

C= N OC NH C = N

OC NH CH 3

S

CH 3

CH 3

(Oxamyl or Methomyl)

CH 3

N -

CH 3 NH 2
+ CH 3 NHCOO -

R C CO

S CH 3

RC N + CH 3 SH

N-Methylcarbamates undergo successive oxidation in soil and are converted to CO 2 as
the final product. Aldicarb or thiobencarb undergoes oxidative degradation easily in chlo-
rinated water.
During the studies on carbamate pesticides, the author himself found that the degrada-
tion of carbamate pesticides increased with soil organic matter, soil surface area, percent-
age clay content, temperature, and soil-moisture content (Bansal 2005), amending the soil
with farm yield manure (FYM). It was also found that degradation increased with pH and
was faster in alkaline medium (Bansal 2004).